Information resources, such as webpages and pages of mobile applications, can include both first-party content items and third-party content items. First party content items or primary content is content that is provided by a first party content provider. Third-party content items, such as online advertisements may be displayed with the first-party content items. In some embodiments, third-party content items can be inserted in third-party content slots that are positioned on the information resource. The layout, arrangement, and selection of third-party content items displayed in an information resource can contribute to a viewer's viewing experience of the information resource.